Mix sugar, cocoa, starch, salt in a bowl. Gradually add the milk until the cornstarch is dissolved. Pour egg yolks into the mass. Put the mixture on a slow fire and cook, stirring constantly, about 8 minutes. Remove from heat and pour the mixture through a sieve into a bowl. Add butter, vanilla and chocolate, stir until smooth. Place the mass in a pudding mold and refrigerate for 3 hours (maximum 3 days). Before serving, whisk the mixture until smooth, divide into 4 portions and decorate with chocolate chips.